The present invention relates generally to clad metal composites used in cookware products and methods for bonding dissimilar metals. More particularly, the present invention relates to a multilayer bonded composite of dissimilar metals for use as the cook surface in griddles and grills of the type used in commercial establishments. Commercial fast food restaurants typically cook a high number of hamburger patties simultaneously on a griddle/grill cook surface. Conventional griddle cook surfaces are made of a heavy piece of carbon steel, on the order of one inch in thickness. This heavy gauge steel material provides a heat sink in an attempt to maintain a uniform high temperature along the griddle surface.
The attainment of a uniform high temperature is important in the commercial setting, to ensure that all food products, particularly hamburger patties, are cooked to a proper degree. Sickness and even death have occurred from E. coli food poisoning in hamburger meat which has been undercooked. According to recent statistics from the Centers for Disease Control, as many as 40,000 people a year may become ill from E. coli 0157:H7 bacteria. Undercooked or raw hamburger has been implicated in nearly all documented outbreaks.
In conventional, fast food cooking operations it is not uncommon for the restaurant to use frozen hamburger patties and to place the frozen patty directly on the griddle surface. Naturally, when a frozen patty contacts the griddle surface, heat is extracted therefrom and the griddle cools down in those areas. A similar cool-down occurs when the patties are periodically flipped during the cooking cycle. It is well-known that carbon steel is not one of the better heat conductors and, as such, there is a certain time lag after the surface cools down due to the placement of cold or frozen patties or the flipping operation to return to the proper minimum cooking temperature.
Thus, conventional carbon steel griddle surfaces experience significant time lags for thermal recovery which necessarily reflects on the total time required to thoroughly cook the food product.
In an effort to achieve greater product throughput, it is common commercial practice to flip hamburger patties to a hot spot on the griddle, i.e., a location which was previously unoccupied. In order to ensure food preparation safety, such a cooking technique requires proper training of personnel but does not eliminate the likelihood of accidental undercooking due to the occasional, inadvertent placement of the food product on a cold spot on a griddle.
Commercial carbon steel griddles require long preheat times prior to use due to the thermal conductivity shortcomings of steel alluded to above. This results in added energy costs. In addition, carbon steel griddle plates typically have a two- to three-inch wide region at the peripheral edge which is lower in temperature and therefore cannot be used for cooking due to lack of heat. Once again, total throughput is adversely affected by this limitation as well as the possible food safety issue due to undercooking if the food product is inadvertently placed near the edges.
Ideally, a commercial griddle plate should possess a hard cook surface such as steel to resist scratching and abrasion, but it should also possess high heat conductivity to reduce cold spot recovery time and preheat time.
Heretofore, there have been attempts to manufacture cooking griddles made from a composite metal plate having improved thermal conductivity properties so as to minimize the thermal recovery time lag caused by cold spots. One such product utilized a stainless steel clad aluminum plate. This attempt at a griddle plate has not been successful due to several inherent problems. Aluminum and stainless steel have significantly different coefficients of thermal expansion which causes a flat composite plate to experience thermal warpage upon heating. In addition, the bond between the stainless steel and the aluminum fails when the product is welded. This is caused by the formation of brittle intermetallic compounds of aluminum at the interface at elevated welding temperatures.
A further prior cooking griddle is depicted in U.S. Pat. No. 5,227,597 which discloses an induction heating surface employing a copper core with a nickel iron outer cladding. The copper core is relatively thin--0.100 inch--to provide localized heating and cooling. There is no disclosure as to how a bond is effected between the nickel iron and the copper core, although one common method to achieve such a bond involves brazing the dissimilar metal sheets. Such a bonding method does not produce a particularly suitable joint for a cooking surface since the thermal conductivity between the core and the outer surface layers is disrupted by the brazed joint which contains lower conductivity brazing fluxes, etc. Also, at cooking temperatures the brazed construction has been known to delaminate, which causes even more thermal conductivity problems due to the presence of insulating air spaces developed within the delaminated joints.
It has long been known that copper is difficult to roll bond to other metals because the copper oxide layer which forms on the surface upon exposure to the atmosphere is very ductile and prohibits contact with the underlying bare metal which is necessary to achieve diffusion bonding during roll bonding, for example.
An early method of joining a core of a 102 copper alloy to a copper nickel alloy, known as cupronickel, has also been employed to make composite stock for coinage. This method involved the so-called vacuum pack technique of roll bonding. A stack of starting plates comprising two outer plates of cupronickel with a core plate of 102 copper alloy had their peripheral edges welded to a border plate. The surfaces of the copper alloy plate were first abraded to remove the oxide layer. The edge welded stack was then evacuated by a vacuum pump and sealed to provide an oxygen-free environment for subsequent roll bonding. Thus, no oxide layer could form on the copper plate at rolling temperature due to the existence of a vacuum in the welded stack of plates. The copper alloy core and the cupronickel outer layers possess similar hot working temperatures to permit relatively easy rolling. The main problem in the above method, however, resides in the difficulty in achieving a sound vacuum pack which is imperative for proper roll bonding. Due to the inability to achieve uniformity, this method of bonding is very labor intensive and expensive.
An ideal cooking griddle would employ a core of copper or copper alloy with an intimately bonded outer layer of a corrosion-resistant material such as stainless steel. Other advantageous griddle plates would also employ the copper core but may employ a cooking surface of titanium or carbon steel.
To the best of my knowledge, it has heretofore not been possible to conventionally roll bond a composite plate of stainless steel with a copper core due to the difficulty in hot rolling these two materials. The hot working temperature of stainless steel approaches the melting point of copper, which makes the rolling process very difficult to commercially practice. The previously described vacuum pack method of initially forming a starting composite prior to rolling is also prohibitively costly, thus making a stainless steel clad copper core product commercially unattainable. The present invention provides a method for diffusion bonding copper to other dissimilar metals, such as stainless steel, carbon steel and titanium, and for subsequently rolling the bonded composite to provide an improved griddle plate in an economically feasible manner.
The present invention not only solves the problems confronted by commercial fast food establishments but has also overcome the limitations in the composite metals art by providing a clad metal composite griddle plate which offers several important advantages over the existing carbon steel griddle. The griddle plate of the present invention provides a cooking surface having far more rapid thermal recovery than that of the conventionally used carbon steel griddle plates which translates into greater food product throughout. In addition, the present invention provides a composite griddle plate material which practically eliminates cold spots on the cooking surface to greatly decrease the likelihood of accidental undercooking so as to markedly improve and safeguard health safety in the fast food industry.
Still further, the present invention provides a griddle which maintains a constant cooking temperature from edge to edge, thus eliminating the cold edge problems present in the prior art carbon steel griddle plates. In addition, the invention provides a composite griddle plate that can be bent, cut, formed or welded using the same methods employed for carbon steel and which does not form harmful, brittle intermetallics which prevent the welding of prior art heat conductive composites containing an aluminum core. Still further, the present invention provides a composite griddle plate which, upon heating, does not thermally warp or delaminate due to the similarities in the coefficients of thermal expansion of the materials employed in the composite.
In addition, the composite griddle of the present invention minimizes the initial heat-up time as well as the heat recovery time between cooking cycles to provide decreased energy costs and increased labor efficiencies.